It's ridiculous, but in line with the travel industry as a whole (picking and choosing what they will allow you to do to suite them, no matter what the real inventory is).

Hyatt does this with redeeming pts and pt+cash too -- sometimes only "queens" is allowed, sometime only "kings". Today, I rebooked a pt+cash stay this summer because they opened availability to Kings (when I originally booked, only 'two twins' was allowed for pts+Cash).

I don't understand the games they play. Push forward your complaint, but this isn't out of the ordinary in any way.